U.S. patent number 5,071,906 [Application Number 07/614,964] was granted by the patent office on 1991-12-10 for polarizing film and process for the production of the same.
This patent grant is currently assigned to Unitika Ltd.. Invention is credited to Minoru Nishinohara, Masami Shiozawa, Chikafumi Tanaka.
United States Patent |
5,071,906 |
Tanaka , et al. |
December 10, 1991 |
Polarizing film and process for the production of the same
Abstract
A polarizing film comprising a uniaxially stretched film
containing a polyvinyl alcohol having a degree of polymerization of
at least about 2,500, and at least one polarizing agent selected
from iodine and a dichroic dye, as well as a process for the
production of the same. This polarizing film is superior in heat
resistance and moist heat resistance to conventional films and has
excellent optical properties, such as a high polarizing coefficient
and transmittance.
Inventors: |
Tanaka; Chikafumi (Kyoto,
JP), Shiozawa; Masami (Kyoto, JP),
Nishinohara; Minoru (Kyoto, JP) |
Assignee: |
Unitika Ltd. (Hyogo,
JP)
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Family
ID: |
15850684 |
Appl.
No.: |
07/614,964 |
Filed: |
November 16, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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215142 |
Jul 5, 1988 |
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Foreign Application Priority Data
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Jul 3, 1987 [JP] |
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62-167492 |
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Current U.S.
Class: |
524/557;
264/289.6; 264/288.4; 524/438; 264/1.34 |
Current CPC
Class: |
C08J
7/06 (20130101); G02B 5/3033 (20130101); C08J
2329/04 (20130101) |
Current International
Class: |
C08J
7/00 (20060101); C08J 7/06 (20060101); G02B
5/30 (20060101); C08L 029/04 (); B29D 011/00 () |
Field of
Search: |
;524/438,557
;264/288.4,289.6,1.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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55-90926 |
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Jul 1980 |
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JP |
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58-68008 |
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Apr 1983 |
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JP |
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60-16903 |
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Apr 1985 |
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JP |
|
Other References
The Merck Index, 9th Edition, p. 946..
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Primary Examiner: Michl; Paul R.
Assistant Examiner: Szekely; Peter
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas
Parent Case Text
This is a continuation of application Ser. No. 215,142, filed July
5, 1988.
Claims
What is claimed is:
1. A polarizing film comprising a uniaxially stretched film
containing a polyvinyl alcohol having a degree of polymerization of
about 2,500 to about 10,000, and at least one polarizing agent
selected from iodine and a dichroic dye.
2. The polarizing film as claimed in claim 1, wherein said
polyvinyl alcohol has a degree of polymerization of at least about
4,500.
3. The polarizing film as claimed in claim 1, wherein said
polyvinyl alcohol has a degree of polymerization of about 6,000 to
10,000.
4. The polarizing film as claimed in claim 1, wherein said
polarizing agent is dissolved in said polyvinyl alcohol solution
before formation of the polarizing film.
5. The polarizing film as claimed in claim 1, wherein said
polarizing film is polarized by forming a film with a mixture
containing about 0.2 to 3% by weight of at least one polarizing
agent selected from a group consisting of iodine and dichroic dye
dissolved in a polyvinyl alcohol solution.
6. The polarizing film as claimed in claim 1, wherein said
polyvinyl alcohol has a degree of polymerization of about 4,980 and
a degree of hydrolysis of about 99.8%, and wherein said polarizing
film is polarized by coating a film with a solution containing a
polarizing agent selected from the group consisting of
an aqueous solution containing 1% by weight of iodine and 5% by
weight of potassium iodide;
a dyeing liquor of 0.05% Congo Red (C.I. Direct Red 28);
an aqueous solution containing 0.05% by weight of iodine and 0.25%
by weight of potassium iodide;
methanol containing 1% by weight of iodine; and
methanol containing 0.5% by weight of methylene
4-aminoxanthopurpurin.
7. The polarizing film as claimed in claim 1, wherein said
polyvinyl alcohol has a degree of polymerization of about 3,250 and
a degree of hydrolysis of about 99.6% and wherein said polarizing
film is polarized by coating a film with a solution containing
0.03% by weight of iodine and 0.20% by weight of potassium
iodide.
8. The polarizing film as claimed in claim 1, wherein said
polyvinyl alcohol has a degree of polymerization of about 6,740 and
a degree of hydrolysis of about 99.2% and wherein said polarizing
film is polarized by coating a film with methanol containing 0.5%
by weight of methylene 4-aminoxanthopurpurin.
9. The polarizing film as claimed in claim 6, wherein said
polarizing agent comprises methanol containing 1% by weight of
Congo Red (C.I. Direct Red 28).
10. The polarizing film as claimed in claim 1, wherein said
polyvinyl alcohol has a degree of hydrolysis of 95 to 100 mol%.
Description
FIELD OF THE INVENTION
This invention relates to a polarizing film and a process for the
production of the same. More particularly, it relates to a
polarizing film including a uniaxially stretched film of polyvinyl
alcohol (hereinafter, "PVA") having a high degree of polymerization
as a base, with significantly improved heat resistance and moist
heat resistance as well as excellent optical properties; and a
process for the production of the same.
BACKGROUND OF THE INVENTION
Known polarizing films include a uniaxially stretched PVA film on
which iodine and/or dichroic dye(s) are adsorbed. A PVA film
employed in this polarizing film as a base generally comprises PVA
of a low degree of polymerization, such as 2,000 or below. This
polarizing film has been widely employed, for example, in liquid
crystal displays of, e.g., pocket electric calculators, watches,
word processors, liquid crystal printers, liquid crystal color
TV's, various instruments and automotive instrument panels.
Further, it is inserted into laminated glass to thereby form an
anti-glare film for, e.g., sunglasses or ski goggles. Although this
polarizing film exhibits an excellent polarizing coefficient, it
has a poor water resistance and moist heat resistance and lacks
heat resistance since it contains a hydrophilic polymer as the
base. Therefore, this polarizing film frequently suffers from the
deterioration of optical properties, such as a decrease in the
polarizing coefficient, when exposed to high temperature and/or
humidity. With the development of the electronic industry, the
application range of liquid crystal displays has been more and more
extended. Accordingly, a polarizing film is urgently required which
has a high water resistance, heat resistance and moist heat
resistance in addition to excellent optical properties such as a
high polarizing coefficient and transmittance.
Under these circumstances, there has been proposed a uniaxially
stretched polyester film of improved heat resistance and moist heat
resistance which is obtained by incorporating a dichroic dye into a
polyester, melt-extruding the resulting mixture and then stretching
the same (cf. JP-A-58-68008, JP-A-58-124621, JP-A-60-125804,
JP-A-61-65202, JP-A-62-141503 and JP-A-62-145204) (the term "JP-A"
as used herein means an "unexamined published Japanese patent
application"). Although this polarizing film is superior in heat
resistance and moist heat resistance to a conventional PVA
polarizing film, the polarizing coefficient thereof is still
insufficient. For this reason, it has been seldom used in practice,
compared with PVA polarizing films.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a polarizing film
having an improved heat resistance and moist heat resistance.
A further object of the invention is to provide a polarizing film
having excellent optical properties such as a high polarizing
coefficient and transmittance.
Another object of the invention is to provide a process for the
production of the same.
As a result of extended studies, it has now been found that these
and other objects of the present invention can be achieved by
employing a uniaxially stretched film containing PVA of a high
degree of polymerization as a base film.
Accordingly, the present invention relates to a polarizing film
comprising a uniaxially stretched film containing a PVA having a
degree of polymerization of least about 2,500 and at least one
polarizing agent selected from iodine and a dichroic dye. The
invention also relates to a process for producing a polarizing film
comprising the steps of: (a) dissolving PVA having a degree of
polymerization of at least about 2,500 in a solvent in a
concentration of about 2 to 35% by weight; (b) forming a film from
the resulting PVA solution; and (c) uniaxially stretching the film
obtained to provide a stretched film; provided that at least one
polarizing agent selected from iodine and a dichroic dye is present
in the solution in step (a); or is applied to the film in step (b)
or step (c), or to the stretched film.
DETAILED DESCRIPTION OF THE INVENTION
Now the present invention will be described in greater detail. The
polarizing film of the present invention employs a uniaxially
stretched PVA film as a base. The PVA used in the present invention
should have a degree of polymerization of at least about 2,500,
preferably at least about 4,500 and still more preferably about
6,000 to 10,000. From the viewpoint of the optical properties and
durability cf the polarizing film, it is preferred that the degree
of polymerization of the PVA is as high as possible. However, the
degree of polymerization of the PVA is typically less than about
10,000 in practice, by taking the production cost into
consideration. A polarizing film containing a PVA film wherein the
degree of polymerization of the PVA is less than about 2,500 is
unsatisfactory in optical properties, heat resistance and moist
heat resistance.
The improved heat resistance and moist heat resistance of the
polarizing film of the present invention, which contains PVA of
such a high degree of polymerization as specified above, makes it
highly useful, since the poor heat resistance and moist heat
resistance of conventional ones are serious disadvantages.
Furthermore, the polarizing film of the present invention has
excellent optical properties such as a high polarizing coefficient
and transmittance. In the polarizing film, the transmittance has a
relationship to be contrary to the polarizing coefficient, and the
transmittance and polarizing coefficient are adjusted in accordance
with the dyeing degree obtained by a polarizing agent. The higher
the concentration of the polarizing agent in the polarizing film,
the transmittance of films is reduced but the polarizing
coefficient is increased (the maximum value of the polarizing
coefficient becomes 100%). On the other hand, the lower the
concentration of the polarizing agent, the transmittance of films
is increased but the polarizing coefficient is reduced. A highly
efficient polarizing film excels in both of transmittance and
polarizing coefficient. The ideal sets of transmittance and
polarizing coefficient values of the polarizing film are 50% and
100%, respectively. The polarizing film according to the present
invention exhibits, for example, a polarizing coefficient of 100 to
about 98% when the transmittance is about 42 to 45%, preferably a
polarizing coefficient of 100 to about 99% when the transmittance
is about 44 to 48%. Without being bound by theory, it is considered
that this might be due to the fact that the polarizing film of the
present invention is highly stretchable in a single direction and
thus the orientation of the iodine or dichroic dye is improved
thereby.
It is preferred that the PVA used in the present invention has a
degree of saponification of at least about 95% by mol and still
preferably at least about 99% by mol.
The polarizing film of the present invention may be produced by,
for example, the following process.
PVA of a degree of polymerization of about 2,500 or more, as
mentioned above, is dissolved in a solvent to thereby give a
polymer concentration of about 2 to 5% by weight, preferably about
4 to 18% by weight, most preferably about 5 to 12% by weight. When
the concentration of the PVA exceeds about 35% by weight, the
resulting solution becomes highly viscous. Thus, the uniformity of
the solution is lowered and PVA molecular chains would entwine each
other, which lowers the stretchability of the obtained film. Thus,
it is undesirable to employ such a high polymer concentration. When
the concentration of the PVA solution is less than about 2% by
weight, on the other hand, a film can be hardly formed.
As one embodiment of the production process according to the
present invention, the step of preparing the solution, in which a
polarizing agent is dissolved in the PVA solution, is included In
this step, the amount of the polarizing agent used varies depending
upon a kind of polarizing agents, but its amount preferably ranges
from about 0.2 to 3 wt% per the weight of the PVA solution. The
preparation of the PVA solution having the polarizing agent
dissolved therein is preferably carried out at a temperature of
from about 60.degree. to 120.degree. C. When iodine is used as a
polarizing agent in such preparation, it is more preferred that
said preparation is carried out at a lower temperature among the
above temperature ranges of from about 60.degree. to 120.degree. C.
to prevent the sublimation of iodine. As the method for preparing
the abovementioned solution, for example, a method comprising steps
of adding the polarizing agent in the following solvents for
dissolving PVA, dissolving the polarizing agent therein under
stirring, and then adding PVA in the resulting solution, followed
by dissolving PVA therein under stirring, can be employed.
Examples of the solvent for the PVA include organic solvents such
as dimethyl sulfoxide (hereinafter, "DMSO"), dimethylformamide,
acetone, methyl alcohol, n-propyl alcohol, ethylene glycol and
propylene glycol. Any one of these solvents or a mixture thereof
may be used. It is further possible to use a mixture of one or more
of these organic solvents and an aqueous solution of one or more of
inorganic salts such as calcium chloride or lithium chloride. It is
also possible to use water with the one or more solvents. Among
these organic solvents, DMSO is particularly preferred and a
solvent mixture of DMSO and water is also preferred. In the latter
case, the mixture contains from about 55% by weight up to almost
100% by weight of DMSO and from almost 0% by weight up to about 5%
by weight of water. In this case, it is preferred to contain about
70 to 98% by weight of DMSO. When a solvent mixture containing DMSO
and water at a ratio of about 90 to 96% by weight of DMSO and about
10 to 4% by weight of water is employed as the solvent, a PVA
solution of a relatively high concentration would not be solidified
within a relatively short period of time required for the formation
of a film at a relatively low temperature, i.e., around room
temperature (about 15.degree. to 35.degree. C.). This might be
caused by the fact that the high compatibility of the PVA with the
solvent would enhance the solubility of the PVA. Therefore, the use
of such a solvent mixture as defined above is advantageous in that
the PVA solution would not be solidified until it is introduced
into a coagulating solution, and thus a uniform film with little
variation in thickness can be formed at room temperature.
According to the process of the present invention, a film is
produced from the PVA solution in the following manner.
For the formation of a film, the PVA solution is discharged into
air or an inert atmosphere such as a nitrogen atmosphere through,
for example, a slit outlet to thereby form a liquid film of PVA
solution. Then, it is introduced into a coagulating solution
wherein a solidified film is formed. Alternately, the PVA solution
may be formulated into a liquid coating in the above atmosphere by
using, for example, a roll coater. It is further possible to
directly introduce the PVA solution into the coagulating liquor to
thereby form a solidified film, without discharging the solution
into any atmosphere. Alternately, a PVA film may be produced by
forming a liquid film of PVA solution in the atmosphere, once
cooling the liquid film with a cooling medium such as carbon
tetrachloride, decalin, paraffin or trichloroethylene to thereby
allow the same to be solidified, desolvating the same in a
desolvating solution, and then drying. Cooling medium having lower
boiling point such as carbon tetrachloride and trichloroethylene
would not need the desolvating process. The method for the
preparation of the PVA film is described, for example, in Polyvinvl
Alcohol-Properties and Applications, edited by C.A. Finch, Croda
Polymers Ltd., Luton, John Wiley & Sons, pages 378-379
(1973).
Examples of the coagulating solution or desolvating solution
include alcohols such as methanol, ethanol, propanol, isopropanol
and butanol, and acetone. Any one of these materials or a mixture
thereof may be employed.
The liquid film of the PVA solution may be typically formed at a
temperature of the PVA solution of about 10.degree. to 120.degree.
C., though the temperature may vary depending on the employed
solvent. As described above, a uniform liquid film of PVA solution
showing little variation in thickness can be obtained at a
relatively low temperature, i.e., around room temperature of
15.degree. to 35.degree. C., by using a solvent containing 90 to
96% by weight of DMSO and 10 to 4% by weight of water. This liquid
film may be, either directly or after discharging in an atmosphere,
introduced into a coagulating solution or cooled with a cooling
medium, introduced into a desolvating solution to thereby form a
solidified film, and then dried.
The unstretched film thus obtained is then stretched in a single
direction up to about 5-fold or more, preferably about 7-fold or
more in the range of avoiding breakage. When the stretching ratio
is less than about 5-fold, the resulting polarizing film would be
unsatisfactory in polarizing coefficient and transmittance.
Although the higher stretching ratio the better, the upper limit is
approximately 20-fold from a practical viewpoint. Most preferred
stretching ratio may be about 8 to 15-fold. The stretching rates is
about 10 to 300 %/min of original length, preferably about 50 to
200 %/min. The wet-stretching of PVA film is described, for
example, in Y. Oishi and K. Miyasaka, Polymer Journal, 19(3),
331-336 (1987).
Either wet- or dry-stretching may be employed.
In the case of wet-stretching, the unstretched film is
preliminarily allowed to swell in, for example, a swelling liquor
at room temperature, i.e., about 15.degree. to 35.degree. C. or
slightly higher, at which the film is not dissolved. Then, the
stretching is carried out in a liquid at about 15.degree. to
60.degree. C. When the stretching temperature is extremely low, it
is impossible to achieve a sufficient stretching ratio. Thus, it is
preferred to adjust the stretching temperature at least above the
lower limit as defined above, i.e., about 15.degree. C. When the
stretching is to be effected in a liquid at a high temperature,
e.g., about 55.degree. to 60.degree. C., it is preferred to carry
out preliminary stretching at around room temperature to thereby
prevent the PVA film from being dissolved at a higher temperature,
and then elevating the temperature while applying tension thereto,
depending on the degree of polymerization of the PVA.
Examples of the liquid in which the swelling or stretching may be
carried out include a dyeing solution containing polarizing agent
and dyeing aid such as inorganic salts, and aqueous solution of,
e.g., about 0.5 to 3% by weight boric acid which is used as a
crosslinking agent.
In the case of dry-stretching, the procedure is carried out at a
temperature of about 100.degree. to 250.degree. C. an atmosphere
such as air or, preferably, nitrogen gas. When the stretching
temperature is lower than about 100.degree. C., it is sometimes
impossible to establish a sufficient stretching ratio. When it
exceeds about 250.degree. C., on the other hand, the film might
melt at the subsequent thermal treatment step, at which a higher
temperature than that of stretching is needed.
The film thus uniaxially stretched may be then subjected to a
thermal treatment. This thermal treatment, which may be carried out
in air or in an inert gas atmosphere, further improves the
durability including dimensional stability, heat resistance and
moist heat resistance of the stretched polarizing film. This
treatment may be preferably effected at about 180.degree. to
260.degree. C., preferably about 200.degree. to 240.degree. C., for
about 0.1 to 10 minutes. It is preferred that this treatment is
effected at a temperature higher than the stretching temperature.
It is undesirable to carry out the thermal treatment at a
temperature lower than about 180.degree. C., since the degree of
crystallinity of the film is hardly elevated in this case and thus
the durability of the film is scarcely improved. On the other hand,
it is also undesirable that the temperature exceeds about
260.degree. C., since the film might melt even though it is treated
under applying tension thereto.
It is preferable that the above thermal treatment be performed
under a tension applied to the film sufficient to maintain the same
length achieved after stretching, and to avoid any looseness.
Iodine and/or dichroic dye(s) may be added to the film in a
conventional manner. That is to say, they may be added in any step
before or during the stretching step following the dissolution of
the PVA in the solvent. Alternately, they may be added in a
separate step after the completion of the stretching.
The procedure for the addition of iodine is not particularly
restricted. For example, the film may be immersed in an aqueous
solution of a mixture of iodine and potassium iodide. This aqueous
solution may further contain a crosslinking agent such as boric
acid, borax or glutaraldehyde in order to inhibit the scattering of
the iodine, and a treatment with a crosslinking agent may be
separately carried out after the dyeing.
As the dichroic dye, yellow, orange, blue, purple or red dyes may
be used. Typical examples thereof include direct dyes including
C.I. Direct dyes such as Black 17, 19 and 154, Brown 44, 106, 195,
210 and 223, Red 2, 23, 28, 31, 37, 39, 79, 81, 240, 242 and 247,
Blue 1, 15, 22, 78, 90, 98, 151, 168, 202, 236, 249 and 270, Violet
9, 12, 51, and 98, Green 1 and 85, Yellow 8, 12, 44, 86 and 87,
Orange 26, 39, 106 and 107 and dispersion dyes including C.I.
Dispersion dyes such as Blue 214, Red 60 and Yellow 56. In the
present invention, two or more dichroic dyes may be used together.
Furthermore, iodine and dichroic dye(s) may be used together in the
present invention in order to control the hue.
The iodine and/or dichroic dye(s) may be added as mentioned above
during the preparation of the PVA solution.
When wet-stretching is to be effected, iodine and/or dichroic
dye(s) used as a polarizing agent may be simultaneously added
during the swelling or stretching step. When the iodine is applied
to the swelling or stretching step, the solution of about 0.001 to
3% by weight of iodine and about 0.005 to 15% by weight of
potassium iodide, preferably the solution of about 0.03 to 1% by
weight of iodine and about 0.2 to 5% by weight of potassium iodide,
is usually used. When the dichroic dye is applied to the
abovementioned step, the concentration of the dichroic dye used
preferably ranges from about 0.001 to 3% by weight. The film thus
dyed and stretched is then dried and subjected to the
abovementioned thermal treatment. Only for iodide case, the
temperature must be lowered, or dyeing must be effected after
thermal treatment. Alternately, the stretched and thermally treated
film may be dyed with iodine and/or dichroic dye(s). In this step,
when iodine is applied, about 0.1 to 3% by weight of iodine and
about 0.5 to 15% by weight of potassium iodide are preferably used,
and when dichroic dye is used, about 0.1 to 3% by weight of
dichroic dye is preferably used. As the method for applying iodine
or dichroic dye, a dipping method may be employed. When
dry-stretching and dyeing with an iodine dye are combined, it is
preferred to carry out the thermal treatment after the stretching
to thereby inhibit the sublimation of the iodine at a high
temperature. When a dichroic dye highly resistant to thermal
degradation and sublimation is used, the dye may be preliminarily
added during the preparation of the PVA solution. Alternately, the
film may be dyed prior to the stretching or the dyeing may be
carried out during the final step in this case. Iodine and/or
dichroic dye(s) may be added to the film in a conventional manner,
similar to the case of the wet-stretching. When dyeing is effected
after the thermal treatment in the dry-stretching process, the high
degree of crystallinity of the film makes the dyeing difficult.
Therefore, it is required in this case to prolong the dyeing time
up to, e.g., about 30 minutes to 1 hour or to enhance the
concentration of the iodine up to, e.g., about 0.5 to 2% by weight
and/or dichroic dye(s)
When the abovementioned polarizing agent(s) are applied, a swelling
or stretching step is preferred in the wet-stretching process and
an after-stretching step is preferred in the dry-stretching
process. In either case, it is preferred to carry out the
crosslinking treatment using a boric acid solution in a
concentration of about 0.5 to 15 wt%.
The polarizing film of the present invention can be used alone or
in combination with a support or other layers. For example, an
optically isotropic film having a good transparency such as
polyarylate film, triacetate film or (meth)acrylate can be
laminated on the abovementioned polarizing film.
In the abovementioned laminating treatment, for example, a solution
having adhesives such as isocyanate dissolved in a solvent is
applied on a polyarylate film by means of a roll coater or a bar
coater, the coated film is dried to remove the solvent, and then
the polyarylate film is laminated on the polarizing film by means
of a laminator.
Among these films, the polyarylate film (trade name, "EMBLATE"
manufactured by UNITIKA LTD.) having a thickness of about 50 to 130
.mu.m is particularly preferred. Since the polyarylate film is an
optically isotropic and heat resistant film having properties of
hardly passing a moisture, it is preferably used as a protective
film for the polarizing film.
The polarizing film of the present invention containing PVA of a
high degree of polymerization has a significantly improved heat
resistance and moist heat resistance, compared with conventional
films which are seriously deficient, as well as an excellent
polarizing coefficient and transmittance. In addition, the
polarizing film of the present invention contains PVA of a high
degree of polymerization and is used together with an organic
solvent. Thus, a dispersion dye, which extends the selection range
of dyes, can be used making the polarizing film of the present
invention quite advantageous.
The polarizing film of the present invention is widely applicable,
not only to liquid crystal displays of, for example, work stations
of office automation systems, liquid crystal TV's, automotive
instrument panels, which should have a high moist heat resistance,
and various instruments, but also to filters of panel photographs,
sunglasses, residential or building windows and various
sensors.
According to the production process of the present invention, a
polarizing film, which can be stretched at a high ratio compared
with conventional ones and has excellent optical properties as well
as a high heat resistance and moist heat resistance, can be
efficiently produced by using a commercially available PVA of a
degree of polymerization of at least about 2,500, preferably at
least about 4,000.
To further illustrate the present invention, the following specific
Examples are provided, but the present invention is not to be
construed as being limited thereto. Unless otherwise indicated, all
parts, percents and ratios are by weight. In the Examples,
transmittance and polarizing coefficient are determined by the
following methods.
The transmittance of visible light (wavelength: 400 to 700 nm) was
determined with a spectrophotometer. A polarizer was placed in the
incident light side. The transmission axis of a polarizing film
sample was overlapped with the optical axis of the polarizer and
then placed at right angles thereto. Then, the transmittance of the
polarizing film in each case was determined. The algebraic mean of
the data was referred to as the transmittance. According to
JIS-Z-8701 ("JIS" refers to Japanese Industrial Standard), the
transmittance was determined by correcting the visibility over the
visible light range under 2.degree. viewing angle "XYZ colour
specification system" by using a standard illuminant C. When a red
dichroic dye was used, however, the transmittance at 525 nm was
determined. The polarizing coefficient was calculated according to
the following equation by determining the transmittance with axes
parallel (T.parallel.), which was determined by overlapping two
polarizing films in such a manner as to make the axes thereof
parallel with each other, and the transmittance with axes crossed
(T.perp.), which was determined by overlapping the same in such a
manner as to make the axes at right angles to each other:
##EQU1##
The ideal sets of maximum values of transmittance and polarizing
coefficient of a polarizing film are 50% and 100%,
respectively.
Although a polarizing film is usually used in a state having a
protecting film laminated thereon, various properties of
unprotected polarizing films were determined in the following
Examples and Comparative Examples.
Example 1 AND COMPARATIVE Example 1
PVA having a degree of polymerization of 4,980 and a degree of
saponification of 99.8% was dissolved in a solvent mixture of DMSO
and water at a ratio by weight of 95/5 under heating to 80.degree.
C. to thereby give a PVA concentration of 7% by weight. Thus, a PVA
film-forming solution was obtained. This solution was discharged
into a methanol bath via a slit outlet while maintaining the same
at 20.degree. C. to thereby form a film 50 .mu.m thick. The
resulting film was air-dried at room temperature and then
uniaxially stretched at 155.degree. C. up to 7-fold. Then, the
stretched film was subjected to thermal setting in nitrogen
atmosphere at 180.degree. C. for 5 minutes while applying
sufficient tension to maintain the stretch ratio and avoiding any
looseness. Then, the film was immersed in an aqueous solution
containing 1% by weight of iodine and 5% by weight of potassium
iodide for 30 minutes and then in a 3% by weight boric acid bath
for 15 minutes at room temperature. After air-drying, a polarizing
film was obtained. The obtained polarizing film had a thickness of
11 .mu.m, a bluish purple color, a transmittance of 48.3% and a
polarizing coefficient of 99.9%.
This polarizing film was allowed to stand in a thermo-hydrostat at
60.degree. C. and 90% R.H. for five hours and then the
transmittance and polarizing coefficient thereof were determined
again. As a result, the transmittance of the polarizing film was
52.3% while the polarizing coefficient thereof was 93.0%, showing
little change in the properties.
For comparison, the transmittance and polarizing coefficient of a
polarizing film containing PVA of a degree of polymerization of
1,700 and the same polarizing agents as those employed in Example 1
(transmittance: 41.2%, polarizing coefficient: 96.4%, thickness: 13
.mu.m and stretching ratio: 4.5-fold) were similarly determined. As
a result, after the moist heat treatment the transmittance of this
film was 62.7% while the polarizing coefficient thereof was
72.4%.
These results indicate that the polarizing film of the present
invention underwent far less degradation in the transmittance and
polarizing coefficient at 60.degree. C. and 90% R.H., compared with
a conventional one.
Example 2
The procedure of Example 1 was repeated except that the film was
immersed not in an aqueous solution containing 1% by weight of
iodine and 5% by weight of potassium iodide but in a dyeing liquor
containing 0.05% by weight of Congo Red (a dichroic dye). Thus, a
red transparent polarizing film was obtained. The transmittance of
the polarizing film at a wavelength of 525 nm was 36.9% while the
polarizing coefficient thereof was 85.1%.
Example 3
PVA of a degree of polymerization of 4,980 and a degree of
saponification of 99.8% was dissolved in a mixture of DMSO and
water at a ratio by weight of 95/5 under moderately heating at a
low temperature of 80.degree. C. to thereby give a 7% by weight PVA
solution. This solution was applied onto a polyethylene
terephthalate (hereinafter, "PET") film 100 .mu.m thick at
20.degree. C. with a bar coater and immersed in a methanol bath for
ten minutes to thereby form a film. The resulting film was
air-dried at room temperature. Thus, a PVA film was obtained. Then,
the PVA film was stripped from the PET film and was immersed in a
solution of 0.05% by weight of iodine and 0.25% by weight of
potassium iodide at 20.degree. C. for five minutes and uniaxially
stretched up to 9-fold therein. Then, the stretched film was
immersed in a 3% by weight boric acid bath at room temperature for
15 minutes, air-dried and subsequently thermally treated at
65.degree. C. The polarizing film thus obtained (thickness: 7
.mu.m) showed a transmittance of 44.1% and a polarizing coefficient
of 100%.
EXAMPLE 4 AND COMPARATIVE EXAMPLE 2
The procedure of Example 3 was repeated except that the film was
not immersed in the iodine/potassium iodide solution but in
methanol containing 0.5% by weight of methylene
4-aminoxanthopurpurin available, for example, under the trademark
Miketon Fast Pink RL (mfd. by Mitsui Toatsu Chemicals, Inc.) which
was employed as a coagulating solution and a dyeing solution for
the PVA film. Thus, a red and transparent polarizing film was
obtained. The transmittance of this film at a wavelength of 525 nm
was 33.0% while the polarizing coefficient thereof was 98.7%.
It was attempted to dye a commercially available PVA film having a
degree of polymerization of 1,700 and a degree of saponification of
99.9% in methanol containing 0.5% by weight of Miketon Fast Pink
RL, in the same manner as the above process. However, it was
impossible to dye this film.
Example 5
A PVA film-forming solution, which was prepared in the same manner
as the one described in Example 4 except for adding 1% by weight of
iodine, was applied onto a PET film at 20.degree. C. with a bar
coater and then immersed in a methanol bath to thereby form a film.
The obtained film was air-dried at room temperature and then
stretched up to 6-fold in a 3% boric acid solution for 15 minutes
at room temperature. Then, the stretched film was washed with water
and air-dried at room temperature. The polarizing film thus
obtained (thickness: 11 .mu.m) showed a transmittance of 46.2% and
a polarizing coefficient of 99.4%.
Example 6
The procedure of Example 5 was repeated except that the iodine was
replaced by 1% by weight of Congo Red. The dry PVA film thus
obtained was stretched up to 7-fold in the same boric acid solution
at room temperature, washed with water and air-dried. The
transmittance at a wavelength of 525 nm of the polarizing film thus
obtained (thickness: 11 .mu.m) was 30.1% while the polarizing
coefficient thereof was 99.9%.
Example 7
The procedure of Example 4 was repeated except that PVA of a degree
of polymerization of 3,250 and a degree of saponification of 99.6%
was employed to thereby give a PVA film. The obtained PVA film was
stripped from the PET film and was immersed in a solution
containing 0.03% by weight of iodine and 0.20% by weight of
potassium iodide at 20.degree. C. for five minutes and then
uniaxially stretched up to 9-fold therein. Then, the stretched film
was immersed in a 3% by weight boric acid bath at room temperature
for 15 minutes.
The transmittance of the obtained polarizing film (thickness: 5
.mu.m) was 46.0% while the polarizing coefficient thereof was
97.4%.
Example 8
PVA having a degree of polymerization of 6,740 and a degree of
saponification of 99.2% was dissolved in a mixture of DMSO and
water at a ratio by weight of 95/5 at a low temperature of
80.degree. C. to thereby give a 4% by weight PVA solution. This
solution was applied onto a PET film with a bar coater and immersed
in a methanol bath for ten minutes to thereby form a film. The
obtained film was air-dried. Thus, a PVA film was obtained.
Then, the PVA film was stripped from the PET film and treated in
the analogous manner as the one described in the above Example 4 to
thereby give a polarizing film. The transmittance of this film
(thickness: 5 .mu.m) was 46.0% while the polarizing coefficient
thereof was 99.2%.
While the invention has been described in detail and with reference
to specific embodiments thereof, it will be apparent to one skilled
in the art that various changes and modifications can be made
therein without departing from the spirit and scope thereof.
* * * * *